Specifically, the characteristics which can be thus detected or estimated by the application of this invention are:
the maximum uni-axial or deviatoric stress which had been imposed within its recollection span on the mass prior to the sample's extraction;
the maximum amount of sustained continuous stress that the material could withstand yet remain structurally stable;
the rate of change of stress in a rising stress situation;
the distribution of localized stresses within a structural member;
the characterization of different materials based on their acoustic emission response to stress.
The design of an adequate support structure requires two basic data components: the magnitude of the stress to be carried; and the strength of the selected structural material. In a structure such as a mine or major civil engineering excavation, the reliability of both of these data components may be in doubt. Furthermore, the manner, direction and speed of the structure's response to the excavation operations, which obviously reduce the size or alter the strength of the remaining structure, is also in doubt. In order to allow for these doubtful but critical factors, it is normal practice to generously dimension the remaining structure. The effect of this conservative practice may be vastly greater strength than actually needed accompanied by low extraction ratio and elevated production and construction costs. Conversely, if the stresses were to become higher than anticipated, evidence of this fact might remain unnoticed until the situation becomes acutely hazardous.
There are several conventional methods of estimating the current in-situ stress level in a rock mass. The principal method, called over-coring, involves the cementing-in of strain gauges at the location of test and with them, detect the amount of expansion in the rock as it becomes isolated from its surrounding environmental stress by the passage of a much larger diameter core drill which surrounds the immediate region of the test location. This process is characterized by high cost and prolonged time requirement thus making extensive or multiple testing impracticable.
The substance of this invention is a new and novel method of determining, among other things, the maximum in-situ stress that had existed in a rock mass prior to the extraction of a testing sample from it.
A phenomenon of nature is that when many substantially inelastic materials such as, but not limited to, rock, concrete, ceramics, glass, rigid plastics and metals are subjected to compressive stress, they emit ultrasonic pulses known as acoustic emissions (AE). One of the behavioral characteristics of AE is that when the stress on a sample is relaxed from a level of previous maximum and then restressed, there is a significant increase in the rate of AE output as the restressing exceeds the previous maximum level. This charateristic increase of AE at the transition from past experience stress into the new experience range has become known as the Kaiser Effect. This phenomena was originally disclosed in West German patent No. 852,771 issued Oct. 20, 1952 to J. Kaiser. Other patents involving the use of the Kaiser Effect in material stress determinations are U.S. Pat. Nos. 3,774,443 and 4,107,981.
It is a further noted natural characteristic of AE that a sample of material extracted from its native environment, carries in it a Kaiser Effect recollection of the maximum stress to which that environment has been subjected.
It is an object of this invention to provide a process by which the Kaiser Effect recollection in a sample may be recalled, and a specific practical means for accomplishing this.
Another behavioral characteristic of AE, discovered in conjunction and associated with this invention is that if the stress is raised to a new experience level and held there, the AE output continues, but at a substantially exponentially decaying rate. This rate of decay is inversely related to the level of stress as a proportion of the sample's strength. As the level of constant stress approaches the strength limit of the material, the rate of exponential decay becomes slower. At some stress level, the AE will continue unabated and then start increasing. Unless the stress is released, the AE output will accelerate until gross failure occurs. Up to some stress level, the AE output will diminish to substantially zero, but higher than this level it would be likely to start accelerating within a practical future time period, the material is said to be at its Stability Limit Stress.
It is another object of this invention to provide a process by which the level of Stability Limit Stress may be estimated for a particular material, and utilizing the specific facilities provided to determine the Kaiser Effect recollection in a sample of its previously stressed environment.
This invention utilizes two parameters, namely; stress imposed on a specimen, and the AE output from it resulting from that stress. These two parameters, according to this invention, are interrelated. The relationship between them can, therefore, be graphically represented by a series of points, each representing by its position, a combination of the component amounts of AE and stress. These points could be sufficiently close together to merge into a plot line. In this invention, inferences are drawn from the resulting plot of these two simultaneous parameters.
It is recognized that, although a broad variety of materials produce AE when stressed, the plotted relationship of AE versus stress will differ between types of materials. There is also some difference between the plots of the same type of material if the stress is applied in a different direction relative to that material's natural bedding plane or some such directional characteristic. These distinctive differences in the plots of different materials may be manifested in such factors as, but not limited to, the degree of slope, curvature, direction, or abruptness of change in one or more regions of the plot, and the amount of AE total and stress at failure, and the pattern produced during the failure process.
If it were established that AE bears a relationship to strain of the material under stress, the pattern of the relationship between AE and stress could provide valuable insight into the manner in which different types of materials respond to stress.
It is a further object of this present invention to provide a process by which materials can be characterized by their individual patterns of AE responses relative to the stress causing them, giving facility to comparing AE Signature Profiles of different materials. This process will utilize the specific facilities provided to determine the Kaiser Effect recollection in a sample of its previously stressed environment.
It is recognized that the several processes for the preparation of semi-finished specimens may be accomplished through the utilization of commercially available machine tools, in some cases requiring special modifications and adaptations. However, unless such tools are already available for the use of those wishing to prepare such specimens, the obtaining of such commercial machine tools and making the necessary adaptations to them, would involve unjustifiably high capital cost, excessive use of space, the requirement of particular operator skills, would only utilize a small proportion of the capabilities to the equipment, and would be a cost-inefficient method of producing the semi-finished specimens.
It is a further object of this present invention to provide a specialized machine, combining in a single unit, or a plurality of units, the necessary facilities to economically and efficiently produce semi-finished specimens from the provided sample material.